In a face gear, which has annularly arranged gear teeth, lubrication of the gear teeth, which contact an associated gear meshing with the face gear, is an important task. For example, Patent Document 1 discloses a structure in which the surfaces of each tooth have steps for retaining lubricant. Patent Document 2 discloses a structure in which a groove for retaining lubricant is formed in the tooth bottom. Patent Document 3 discloses a structure in which a lubricant retaining portion is formed in a radially outer portion of each gear tooth.
Patent Document 4 discloses a structure of a gear with gear teeth. An associated gear moves along a part of the tooth surface of each gear tooth while making rolling contact. A recess for retaining lubricant is formed in the part contacting the associated gear. That is, when the face gear rotates, the parts contacting the associated gear moves diagonally along each tooth surface of the face gear. Since the lubricant retaining portions are formed at parts of the tooth surfaces that correspond to the paths of the contacting portions, which are moving, each gear tooth can be effectively lubricated.